The present invention relates to ferrous metallurgy and more specifically to clad steel rolled section used to reinforce concrete and a method for producing the same.
Known in the prior art is a method for producing clad steel sheets comprising a basic metal layer made of steel containing, in percent by weight: xe2x89xa61.0 C; from 0.05 to 1.0 Si; from 0.3 to 2.5 Mn, and a clad layer containing Ni+Crxe2x89xa610.0; one or two from the elements: xe2x89xa62.0 Cu; xe2x89xa61.0 Mo; xe2x89xa60.1 SD; xe2x89xa60.1 V; the balance being Fe and minor impurities. The bimetallic clad pack is connected across the edges by welding or by other means, and hot rolled after pre-heating and heat treatment. The composite sheet is heated before rolling to a temperature above 1150xc2x0 C. and held at this temperature (JP, 61-294223, 21.06.88, B 23 K 20/04) [1].
Also known in the prior art is a clad steel rolled section comprising a main steel layer, intermediate layer and surface stainless steel layer, and a method for producing the same. The layers contain a group of alloy elements including Si and Cr, in percent by weight: from 1.5 to 7.0 Si, up to 3.0 Cr, Si/Cr xe2x89xa70.5 in the basic layer; from 0.5 to 7.0 Si, from 2 to 14 Cr, Si/Cr (8.5/Cr-0.5)xc2x10.2 in the intermediate layer, and up to 6.5 Si, from 6.0 to 25.0 Cr, Si/Crxe2x89xa60.4 in the surface layer.
The method for producing the clad steel rolled section comprises the steps of making a bimetallic billet, hot rolling the billet in several passes, and subjecting the billet to thermo-mechanical treatment. (RU, 2155134 Cl, 27.08.2000, B 32 B 15/18) [2].
Most closely approaching the present invention is a method for producing clad steel sheets, wherein a basic strip is made of steel containing, in percent by weight: xe2x89xa60.05 C, from 0.05 to 0.8 Si, from 0.8 to 2.2 Mn, from 0.02 to 0.08 Al, the balance iron, and a clad layer is made of highly corrosion-resistant steel. The strips, united in a pack, are rolled to a predetermined thickness and heat treated, being held at a temperature of 900-1150xc2x0 C. for more than 10 seconds and then cooled with the rate more than 0.5xc2x0 C. per second (JP, 58-150377, 08.03.85, C 21 D 9/52) [3].
Basic problems with the prior art technical solutions include inadequate strength and impact toughness of the produced clad articles, insufficient corrosion resistance, and inferior bond strength between the layers and with concrete.
The object of the present invention is to overcome the above problems and provide a clad steel rolled section used to reinforce concrete, which would exhibit superior mechanical strength and impact toughness, improved corrosion resistance and high-strength bond between the layers and with concrete.
The object of the invention is attained by a clad steel rolled section for reinforcing concrete, produced from a bimetallic ingot comprising a basic material, such as steel containing, in percent by weight, from 1.0 to 5.8 Si and from 0.1 to 5.0 Al, with the proviso that 3.0xe2x89xa6(Si+Al)xe2x89xa66.0, and a surface layer of ferritic or austenitic stainless steel containing a group of alloy elements including Cr and Ni, by hot rolling the ingot to an intermediate billet and then to a rolled section, and subjecting the rolled section to heat treatment.
The basic and surface layers preferably comprise one or more components selected from the group including: manganese, molybdenum, tungsten, vanadium, copper, titanium, niobium, zirconium, yttrium, rare-earth metals (REM), nitrogen and carbon, in percent by weight, in the basic layer: up to 3.0 nickel; up to 3.0 chromium; up to 3.0 manganese; up to 1.0 molybdenum; up to 0.5 tungsten; up to 0.7 vanadium; up to 0.5 copper; up to 0.3 titanium; up to 0.5 niobium; up to 0.5 zirconium; up to 0.2 yttrium; up to 0.2 REM; up to 0.5 nitrogen; up to 0.8 carbon; the balance being iron and impurities, and in the surface layer: up to 34 nickel; up to 25 chromium; up to 6.5 silicon; up to 4.0 aluminium; up to 6.0 manganese; up to 6.1 molybdenum; up to 4.0 tungsten; up to 0.5 vanadium; up to 5.0 copper; up to 1.2 titanium; up to 1.0 niobium; up to 1.0 zirconium; up to 0.5 yttrium; up to 0.5 REM; up to 0.8 nitrogen, up to 0.5 carbon, the balance being iron and impurities.
The surface layer having a relative thickness of up to 20% in the cross-section of the billet is preferably made of a stainless steel with ferritic structure containing, in percent by weight: xe2x89xa63.0 Ni and from 10 to 25 Cr, the basic layer contains Si and Al in the ratio Si/Alxe2x89xa7(1+Ni/Cr), and a diffusion layer between said layers contains the main alloy components in the ratio:
{(Fe)1-x-y(Si,Al)xCry}, where x+yxe2x89xa618. 
The surface layer having a relative thickness of up to 20% in the cross-section of the billet is preferably made of a stainless steel with austenitic structure containing, in percent by weight: from 4 to 34 Ni and from 6 to 25 Cr, the basic layer contains Si and Al in the ratio Si/Alxe2x89xa7(2xe2x88x92Ni/Cr), and a diffusion layer between said layers contains the main alloy components in the ratio:
{(Fe,Ni)1-x-y(Si,Al)xCry}, where x+yxe2x89xa618. 
The object of the invention is also attained in a method for producing a clad steel rolled section for reinforcing concrete, in accordance with the invention comprising the steps of: making a bimetallic bar with a surface layer of a stainless steel having austenitic or ferritic structure; hot rolling in several passes through calibrated rolls, the final rolling being carried out by calibrated rolls with a corrugated surface observing the following relationship:             H      0              ψ      ·      d        ≤  1
where Ho is the maximum depth (height) of corrugations on the roll surface,
d is the bar diameter,
"psgr" is the relative thickness of the surface layer.
The above object is also attained in a method for producing a clad steel rolled section for reinforcing concrete, comprising the steps of: making a bimetallic strip with a surface layer of a stainless steel having austenitic or ferritic structure; hot rolling in several passes by rolls with a plain body, the final rolling being carried out by rolls with a corrugated surface observing the relationship:   0.2  ≤            h      1              ψ      ·              h        0              ≤  1
where h1 is the maximum depth (height) of corrugations on the roll surface,
ho is the thickness of the bimetallic strip,
"psgr" is the relative thickness of the surface layer.
The bimetallic strip is preferably rolled or press formed at a temperature no greater than 1150xc2x0 K to a half-tube section with surface curvature 1/D and wall thickness ho within the relationship:       5    ·                  h        0            D        ≤  1.
It is well known that mechanical properties of clad steel rolled products substantially depend on the strength of the basic material. Under optimal conditions, where the content of Si and Al in the basic layer is within the range defined in claim 1, specifically, in percent by weight: from 1.0 to 5.8 Si and from 0.1 to 5.0 Al, with the proviso that 3.0xe2x89xa6(Si+Al)xe2x89xa66.0), mechanical strength is from 550 to 1300 MPa, and impact toughness is from 1.1 to 3.1 MJ/m2. The clad-to-basic layer bond strength can be considerably improved by disintegration of solid solution Fexe2x80x94Sixe2x80x94Alxe2x80x94Crxe2x80x94Ni and formation of new phase {(Fe, Ni)1-x-y(Si,Al)xCry} with coherent inter-phase boundaries in the diffusion region. In this case the bond strength can be equal to the basic layer strength or even exceed it, this permitting the production of a bimetallic rolled section with entire corrosion-proof surface, e.g. in the shape of a round or square billet with stainless cladding closed over the entire perimeter. The bimetallic billet may be further deformed to a round or square section or to a strip without cladding discontinuities over the entire perimeter, including the end faces of the rolled strip (with edge uncut), and retain corrosion prevention over the entire surface (FIGS. 1 and 2).
If in the produced clad steel rolled products Si and Al content is beyond the upper limit indicated in claim 1, e.g. 6.1 Si and 5.1 Al in percent by weight, mechanical strength decreases to less than 550 NPa at room temperature, and impact toughness decreases to less than 0.9 MJ/m2. This is caused by the reduction in strength and plastic parameters of solid solution Fexe2x80x94Sixe2x80x94Al due to generation of superstructures, such as Fe13Si3, Fe3Si and Fe7Al. The properties of such level fail to meet the requirements imposed upon reinforcing sections.
If Si and Al content is beyond the lower limit, e.g. 0.9 Si and 0.05 Al in percent by weight, mechanical strength of the basic material and inter-layer bond strength decrease to less than 500 MPa and 0.6 MJ/m2, respectively. This is caused by insufficient content of Si and Al in solid iron solution, which enhance strengthening the structure and reduction of iron oxides at the interface between layers. The diffusion boundary which is formed during heating and hot rolling comprises a great amount of oxide films, discontinuities and other structural defects, and deformation often leads to stratification and fractures in the clad layer.
Going beyond the lower limit of the (Si+Al) content in the basic material of the obtained clad rolled products, e.g. at the value of 2.9, leads to mechanical strength reduction below the permissible limit of 500 mPa, while exceeding the upper limit, e.g. at the value of 6.1, leads to impact toughness reduction to less than 1.0 MJ/m3. The aforementioned mechanical properties depend on the level of strengthening the solid solution structure by silicon and aluminium, and the optimal choice of the martensitic transformation temperature range. The rolled products have optimal properties when (Si+Al) content is within the range from 3 to 6 percent by weight (FIG. 3).
During heat treatment of a bimetallic ingot, a diffusion layer is formed in the transition zone between the surface layer and the basic layer, the diffusion layer consisting from FE-based solid solution and alloyed with the main components, such as Si, Al, Cr and Ni, and other components contained in the basic and surface layers, part of which is present in the ingot as impurities (FIGS. 4,5). The diffusion layer is formed as the result of mutual diffusion of the above components, their effect on structural transformations in the diffusion layer being different, e.g. Si, Cr and Al stabilize ferrite, Ni in the amount of up to 3% dissolves in ferrite and in a greater amount stabilizes austenitic areas in the structure. Cr and Ni reduce the martensitic transformation temperature. Si insignificantly increases and Al drastically increases the martensitic transformation temperature. In combination, the processes affect the diffusion layer microstructure formation and, as consequence, the surface/basic layer bond strength.
Another circumstance of significant importance is that the basic layer ferritic structure, which is well stabilized with Si and Al, has a restricted solubility in terms of C content (less than 0.1%), therefore, the remained carbon in the process of heat treatment of bimetallic rolled products is displaced partially to carbide phase and partially to intermediate zone between the layers. As the result, a relatively thin diffusion layer may be enriched in carbon to a significant extent (up to 1% and more), which may precipitate, under unfavorable conditions, as numerous lines of isolated carbide particles over the inter-phase boundaries and grain boundaries, this impairing the surface-to-basic layer bond strength.
The listed alloy components, including nickel, chromium, manganese, molybdenum, tungsten, vanadium, copper, titanium, niobium, zirconium, yttrium, rare-earth metals (REM), nitrogen and carbon may have different purposes in the surface and basic layer. By way of example, Ni and Cr in the surface layer define the structure type and corrosion resistance of the surface. When Ni content is less than 3% and Cr content is from 10 to 25%, the steel has ferritic structure and its surface exhibits high resistance to atmosphere corrosion. At a content from 4 to 34% Ni and from 6 to 25% Cr, the steel structure may be either austenitic/ferritic (duplex) or austenitic, and the steel, consequently, exhibits greater resistance to aggressive gaseous and liquid media, including that at increased temperatures.
In the basic layer, Ni and Cr are contained in silicon ferrite and a part of Cr is in carbides. Ni in solid solution improves hardenability and toughness of the layer.
Manganese in the surface and basic layers improves hardenability of bimetallic steel. It, however, promotes fast grain growth which embrittles the steel structure. Alloying with carbide-forming elements, such as V, W, Ti, Nb, Zr, enhances reduction of microstructure during heat treatment of steel, and affects the redistribution of carbon between disperse carbides and austenitic and martensitic areas of the microstructure (FIG. 6).
By way of example, while preventing formation of chromium carbides over grain boundaries, Mo contributes to prevention of inter-crystallite erosion in the surface layer, improves hardenability, static, dynamic and fatigue resistance, and reduces cold fragility threshold in the basic steel layer. All these factors permit the mechanical properties of the layers to be influenced so that to improve strength and impact toughness of the structure under certain conditions.
Alloying with Y and REM permits the grain growth to be restricted at the step of melting and solidifying the steel, this having a beneficial effect on mechanical properties of the rolled products at subsequent processing. Nitrogen and carbon are the main elements which by reacting with the aforementioned alloy additives form carbide, nitride phase and more often a complex carbonitride non-metallic phase in the form of disperse particles which act as structural barriers at the path of travel of grain boundaries and dislocations, and thereby influence the structure parameters and plastic and strength properties.
Carbon content affects, inter alia, the amount of residual austenite and martensitic phase in the rolled product layer structure, this also dictating mechanical properties of the rolled products under certain conditions.
When producing clad rolled products in which the surface layer having a thickness of up to 20% in the cross-section of the bimetallic ingot consists of a ferritic stainless steel containing 3% Ni and 10-25% Cr, with the Si/Al ratio in the basic layer less than (1+Ni/Cr), Al contained in solid solution increases the martensitic transformation temperature to Such an extent that a part of the diffusion and basic layer volume enriched with carbon and aluminium undergoes martensitic transformation when cooled after hot rolling step (FIG. 7). After martensite disintegration, the basic layer and especially the diffusion layer embrittle, i.e. the dislocations, slipping at plastic deformation, are stopped at martensite areas and here microfractures are often formed in the structure and interlayer bond strength decreases.
When producing clad rolled products in which the surface layer having thickness up to 10% in the cross-section of the bimetallic ingot consists of a ferrite stainless steel containing xe2x89xa63.0% Ni and from 10 to 25% Cr at the optimal Si/Al ratio in the basic layer equal to (Si/Al)xe2x89xa7(1+Ni/Cr), a poorly carbon-enriched ferritic structure is persistently stabilized in the diffusion layer. In this case, in the diffusion and basic layers a considerable part of carbon is retained in solid solution of alloyed ferrite, while the remaining part of carbon precipitates as disperse and uniformly distributed carbides throughout the microstructure of the layers (FIG. 8). In addition, the solid solution Fexe2x80x94Sixe2x80x94Alxe2x80x94Cr disintegrates and a new high strength phase {Fe1-x-y(Si,Al)xCry} forms in the diffusion layer under certain thermal conditions, and when the surface layer thickness is 10-20%, the diffusion layer can grow in depth of the basic and surface layers, and even distribute over the cross-section of the rolled products between the surface stainless layers (FIGS. 9,10). The relative thickness of the surface stainless layer of the initial composition may reduce in this case.
The above factors contribute to the achievement of the highest values of mechanical strength, impact toughness of the basic and intermediate layers, and bond strength of the surface layer, while the surface may retain superior corrosion resistance if the thickness of the surface layer with the initial composition remains sufficient (no less than 5%).
A clad steel rolled section having a surface layer of 6-25%, and Si/Al ratio in the basic layer less than (2xe2x80x94Ni/Cr) promotes reduction in the martensitic transformation temperature in the diffusion layer whereto Cr and Ni additionally diffuse from the surface layer. When the rolled products are cooled, disperse carbide phase precipitates from martensite and residual austenite on numerous defects of the structure, including carbon-enriched inter-phase and grain boundaries (FIG. 1), therefore, the structure of the diffusion layer is prone to embrittlement by disperse hardening. Additionally, when Ni content is more than 5%, and Al content is more than 1% in the diffusion layer, disintegration of the Fexe2x80x94Sixe2x80x94Alxe2x80x94Crxe2x80x94Ni solid solution and formation of the new high strength {(Fe, Ni)1-x-y(Si, Al)xCry} phase are slowed down. The above factors contribute to reduction in the surface-to-basic layer bond strength, and microfractures and stratification may occur during deformation in the tensile stress region of the clad rolled products.
When producing clad rolled products with a coating of an austenitic stainless steel having an optimum Si/Al ratio in the basic layer within the range (Si/Al)xe2x89xa7(2xe2x88x92Ni/Cr), a ferritic structure enriched in Si, Cr and poorly enriched in Ni,Al persistently stabilizes in the diffusion layer. A considerable part of carbon is retained in the solid solution of alloyed ferrite, while the remaining part of carbon precipitates in the form of disperse, uniformly distributed carbides of the {MeCr)23C6} type throughout the microstructure of the diffusion layer (FIG. 8). Furthermore, the Fexe2x80x94Sixe2x80x94Alxe2x80x94Crxe2x80x94Ni solid solution disintegrates in the diffusion layer under certain thermal conditions and a new high-strength {(Fe,Ni)1-x-y(Si, Al)xCry} phase is formed, where at the surface layer thickness of 10-20% the diffusion layer may grow in depth of the basic and surface layer and distribute over the entire cross-section of the rolled products between the surface stainless layers (FIGS. 9,10). The relative thickness ("psgr") of the surface stainless layer with the initial composition may decrease to "psgr"=0.05 (FIG. 1). The above factors enhance mechanical strength and impact toughness of the basic and diffusion layers, and the bond strength of the surface layer to such an extent that the clad rolled products can withstand the most sophisticated deformation shaping without discontinuities in the coating, and thereby maintain the high level of corrosion resistance of the surface.
In the clad rolled products having the stainless austenitic or ferritic steel surface layer greater than 20% in the cross-section, economical efficiency and production practicability are lost, and if the surface stainless layer is less than 10%, Cr and Ni diffusion is insufficient to form a high-strength diffusion layer from the {(Fe,Ni)1-x-y(Si, Al)xCry} phase over the section between the stainless layers. In addition, the remaining thickness (less than 5%) of the stainless layer with the initial composition may be insufficient to adequately prevent corrosion of the rolled products.
A method for producing clad rolled products comprises the steps of: melting steel for a basic and surface layers, assembling a bimetallic ingot, hot rolling the ingot to obtain a billet and heat treating the billet. If the above conditions of the invention are met, the Fexe2x80x94Sixe2x80x94Alxe2x80x94Crxe2x80x94Ni solid solution may disintegrate in the diffusion zone under predetermined thermal conditions and form a new high-strength {(Fe,Ni)1-x-y(Si,Al)xCry} phase, this permitting the surface and basic layers to be very strongly bound. The billet is then hot rolled in several passes by calibrated rolls to a bimetallic bar, then the bar is hot rolled by calibrated rolls with corrugated surface, the maximum depth (or height) H0 of corrugations on the surface of rolls (bar) should not be exceed the ratio ("psgr"xc2x7d) where "psgr" is the relative thickness of the surface layer, and d is the diameter (mm) of the rolled bar (FIG. 1). When the above conditions are met, the obtained corrugations on the surface of the rolled bar provide a quality bond with concrete mix, and the surface corrosion-resistant layer over the entire corrugated section is not thinned more than by half the original relative thickness. This ensures reliable corrosion prevention over the entire surface of the ribbed bar and improves corrosion resistance of the rolled products.
If the bimetallic bar production involves rolling through corrugated rolls having the corrugation height (or depth) beyond the above ratio, particularly, Ho greater than ("psgr"xc2x7d), then an extremely thinned surface layer and even breaks and microfractures in the coating may be observed at some corrugated sections at the maximum non-uniform deformation regions (FIG. 1). In corrosion tests those regions demonstrate obvious corrosion traces, such as rust pitting.
The clad rolled products made from the bimetallic billet may be rolled in several passes by rolls with plain body to a bimetallic strip with a thickness (h0) and a coating ("psgr"), and then rolled on a body with corrugated surface observing the ratio 0.2xe2x89xa6(h1/h0)xe2x89xa61, the corrugated strip is then rolled or press formed at a temperature of xe2x89xa61150xc2x0 K to a half-tube section with a surface curvature (1/D) in the cross-section within the relationship (5xc2x7h0/D)xe2x89xa61, where h1 is the depth (height) of corrugations at the surface of the strip (rolls), ho is the strip thickness, and "psgr" is the relative thickness of the stainless layer (FIG. 2). The above optimal process parameters promote the attainment of maximum strength and corrosion resistance of the strip and the reinforcing section due to restriction of negative impact of nonuniform deformation across the strip section and structural transformations. By way of example, when the ((h1/"psgr"xc2x7h0)xe2x89xa61) ratio is observed, nonuniform deformation of the surface layer, when rolled by corrugated rolls at maximum peak plastic yielding regions of the coating metal, is limited by its thinning no more than by half relative to the region with uniform plastic yielding. That is to say, if the relative thickness of the coating is 10% of the cross-section of the rolled product, then the coating is thinned in peak deformation regions no more than up to 5% of the rolled product cross-section, provided the optimum relationship of the corrugation height on the tool (roll), strip and coating thickness is observed. This provides a continuous, defect-free coating, thereby improving its corrosion resistance. once the strip has been deformed to a half-tube, a considerable nonuniformity in drawing the metal appears in the strip cross-section in different layers, and the greater the bend curvature (or the less the half-tube diameter), the more the external convex surface is deformed by extension relative to the internal surface (FIGS. 12-14). If the (5xc2x7h0/D)xe2x89xa61 ratio is met, the relative extension of the external surface, as compared to the inner surface, is no greater than 25%, this allowing a desired reinforcing section to be formed from the strip of a predetermined thickness without defects and microfractures on the external surface of the half-tube, and the corrosion resistance to be improved thereby. The optimum temperature of forming the half-tube section with process parameters specified by the invention may be from room temperature to 1150xc2x0 K Mechanical strength of the clad rolled products increases with reduction of the deformation temperature due to deformation defects and hardening of the structure, while impact toughness, on the contrary, reduces for the same reasons, but their parameters do not go beyond the permissible standard limits.
When rolling is effected by corrugated rolls, if the corrugation depth on the surface strip is greater than the upper limit (h1/"psgr"xc2x7h0) greater than 1, the surface layer in maximum deformation nonuniformity regions may be thinned more than by half, and microfractures may be observed in the coating. Obvious corrosion traces such as rust pitting can be revealed in this regions in corrosion tests.
When rolling is effected by worn rolls, if the corrugation height on the roll surface is less than the lower limit (h1/"psgr"xc2x7h0) less than 0.2, the obtained corrugation depth and structure on the reinforcing section is insufficient to provide a required bond strength and adherence of concrete mix.
Forming the half-tube at a temperature above 1150xc2x0 K leads to excessive softening and embrittlement of the basic and surface layer structure of the rolled products due to enrichment of the inter-phase and grain boundaries with carbide phase, coagulation, carbide and martensite phase growth, and increased grain size. This impairs mechanical strength and toughness of the clad rolled products.
Forming the half-tube with a surface curvature exceeding the (1/5h0) ratio results in great plastic deformation nonuniformity over the strip section ( greater than 25%), which leads to appearance of microfractures in the coating on the external (convex) half-tube surface. Corrosion traces, such as rust pitting, appear on the convex surface in corrosion tests.